The present invention relates generally to catalytic bodies and more specifically to catalytic bodies for use as cathodes in an electrolytic cell.
The electrolytic decomposition of alkali metal chlorides has long been practiced by the chlor-alkali industry for the production of chlorine gas, caustic, and hydrogen gas. The major components of the cell in which such electrolysis takes place usually includes an anode and a cathode which are in contact with an electrolytic solution, and a diaphragm or membrane separator in the cell to separate the anode and cathode and their reaction products. In operation, the electrolyte, such as sodium chloride or potassium chloride, is continually fed into the cell and a voltage is applied across the anode and cathode. This produces electrochemical reactions which take place at the anode or cathode to form the desired products.
The particular materials utilized for the cathode and anode are important since they respectively provide the necessary catalysts for the reactions taking place at the cathode and anode. The electrolyic solution reacts at the anode to evolve chlorine gas: 2Cl.sup.- .fwdarw.CL.sub.2 +2e.sup.-. The electrolytic solution reacts at the cathode to produce an alkali metal hydroxide or caustic such as sodium hydroxide, and evolve hydrogen gas: H.sub.2 O+2e.sup.- .fwdarw.H.sub.2 +2OH.sup.31 . The role which the cathode catalyst M plays in evolving hydrogen is shown by the following equations: EQU M+H.sup.+ .fwdarw.MH EQU 2MH.fwdarw.H.sub.2 +M+M.
The applied voltage required to produce the above reactions is the sum of the decomposition voltage (thermodynamic potential) of the compounds in the electrolyte being electrolized, the voltage required to overcome the resistance of the electrolyte and the electrical connectors of the cell, and the voltage required to overcome the resistance to the passage of current at the surface of the anode and cathode (charge transfer resistance). The charge transfer resistance is referred to as the overvoltage. The overvoltage represents an undesirable energy loss which adds to the operating costs of the electrolytic cell.
The reduction of the overvoltage at the cathode to lower operating cost of the cell has been the subject of much attention in the prior art. More specifically, the attention has been directed at the reduction of overvoltage caused by the charge transfer resistance at the surface of the cathode due to catalytic inefficiencies of the particular cathode materials utilized.
The cathode overvoltage losses can be quite substantial in chlor-alkali cells. For example, for mild steel cathodes, the cathode material most commonly used by the chloralkali industry, the charge transfer resistance is on the order of 270 mV to 450 mV at one set of typical operating conditions, e.g., electrolyte temperature of 80.degree. C. and current density of 1 KA/m.sup.2. Such cells are used to annually produce a significantly large amount of product and hence the total electrical energy consumed amounts to a very substantial sum especially in view of todays high energy costs. Such a large amount of energy is consumed that even a small savings in the overvoltage such at 30-50 mV would provide a significant reduction in operating costs. Furthermore, due to the trend of rapidly rising costs for electrical energy, the need for reduced overvoltages takes on added importance since the dollar value of the energy to be saved continually is increasing.
Because of decreasing supplies of fossil fuels, the production of hygrogen by electrolysis has taken an increased importance as a potential source of fuel. While hydrogen is a relatively low cost fuel, petroleum based fuels are presently less expensive. One way to make hydrogen more cost competitive is by reducing the energy involved in its production. This can be accomplished in an electrolytic cell by reducing the overvoltages at which such cells operate. Hydrogen produced by electrolysis presently is primarily used to meet the needs of users requiring a very high grade hydrogen. A reduction in overvoltages would provide a further economic advantage over other hydrogen production methods as well as conserving energy.
As stated before, the cathode material which is most commonly used in the chlor-alkali industry and also by the water electrolysis industry is mild steel. Mild steel is utilized because of the low cost of this material and its relative stability in the caustic environment of the electrolyte. Nickel is another material which has also been put to considerable industrial use as a cathode material for hydrogen evolution. Nickel cathodes, however, while somewhat more stable in caustic, exhibit even greater overvoltages than mild steel. Nevertheless, the excessive overvoltages provided by mild steel and nickel cathodes have been reluctantly tolerated by the industry since an acceptable alternative cathode material has not been available and the cost of electrical power until recently was not a major cost consideration.
Mild steel as well as other materials proposed for use as a catalytic material for cathodes for an electrolytic cell, have generally been limited to materials which are substantially crystalline structures. In a crystalline material the catalytically active sites which provide the catalytic effect of such materials result from accidently occurring, surface irregularities which interrupt the periodicity of the crystalline lattice. A few examples of such surface irregularities are dislocation sites, crystal steps, surface impurities and foreign adsorbates.
A major shortcoming with basing the cathode materials on a crystalline structure is that irregularities which result in active sites typically only occur in relatively few numbers on the surface of a crystalline material. This results in a density of catalytically active sites which is relatively low. Thus, the catalytic efficiency of the material is substantially less than that which would be possible if a greater number of catalytically active sites were available for the hydrogen evolution reaction. Such catalytic inefficiencies result in overvoltages which add substantially to the operating costs of the electrolytic cells.
One prior art attempt to increase the catalytic activity of the cathode was to increase the surface area of the cathode by the use of a "Raney" nickel cathode. Raney nickel production involves the formation of a multi-component mixture, such as nickel and aluminum, followed by the selective removal of the aluminum, to increase the actual surface area of the material for a given geometric surface area. The resulting surface area for Raney nickel cathodes is on the order of 100-1000 times greater than the geometric area of the material. This is a greater surface area than the mild steel and nickel cathodes discussed above.
One process for forming a Raney nickel catalyst is described in U.S. Pat. No. 4,116,804. The process involves plating and flame spraying layers of nickel and aluminum respectively, on an electrode substrate, followed by heating the layers at a temperature of at least 660.degree. C. to cause inter-diffusion of the metals. The inter-diffused aluminum is then leached out to give a high surface area nickel coating which exhibits an initial overvoltage which is less than nickel catalysts having a relatively smooth surface.
The Raney nickel catalyst is very unstable, because it is quite susceptible to oxidation in ambient air and consequently must be protected from contact with air when not submersed in the electrolytic cell. Raney nickel cathodes also lack mechanical stability during hydrogen evolution. The degradation reduces the operating life of Raney nickel cathodes and thus they have not been widely accepted for industrial use. Furthermore, the process for producing Raney nickel is relatively costly due to the expense of the various metallurgical processes involved.
Another prior art approach to lower the overvoltage of cathode catalysts has been centered around the use of materials which are inherently better catalysts than mild steel or nickel. Crystalline compositions including noble metals such as platinum, palladium, ruthenium and the like can provide catalysts which exhibit lower overvoltages during utilization as a cathode catalyst, but these materials have other major drawbacks which have prevented a widespread acceptance by industrial users of electrolytic cells. First, these materials are quite expensive, relatively scarce and are usually obtained from strategically vulnerable areas. Platinum catalyst cathodes, for example, when used in an industrial electrolytic cell initially provide low overvoltage at a high cost which renders such materials unsuitable for commercial electrolysis. Another drawback is that once placed into operation in an electrolytic cell, further degradation problems arise since the noble metal materials are quite susceptible to "poisoning".
Poisoning occurs when the catalytically active sites of the material become inactivated by poisonous species invariably contained in the electrolytic solution. These impurities can, for example, include contaminants contained in the electrolyte such as the impurities normally found in untreated water, including calcium, magnesium, iron and copper. Once inactivated such sites are thus no longer available to act as a catalyst for the desired reaction. The use of noble metal containing cathode catalysts other than platinum have also been attempted. These materials have been found to be quite susceptible to poisoning and thus unacceptable for industrial use.
Other attempts have been made to develop materials which offer an improvement upon the mild steel and nickel catalysts commercially used. For example, electrodes made of steel and the like, have been coated by electroplating the same with various materials providing crystalline coatings thereon. While such electrodes provided somewhat reduced hydrogen overvoltages when operated in a chlor-alkali cell, they were subject to corrosion and degradation problems. U.S. Pat. Nos. 4,033,837 and 4,105,531 disclose electroplating an alloy of nickel (80-20%), molybdenum (10-20%) and vanadium (0.2-1.5%) on a conductive electrode to provide a material for use as a chlor-alkali cathode. This material had a somewhat lower overvoltage than uncoated steel, but also suffered from degradation problems.
U.S. Pat. No. 4,080,278 discloses cathode electrodes for an electrolytic cell coated with a compound of the general formula A.sub.x B.sub.y O.sub.z where A is an alkali or lathanide metal, B is chosen from the group: Ti, W, Mo, Mn, Co, V, Nb, Ta; and oxygen. The compound is mixed with a binder metal and coated on an electrode base using techniques that include plasma and flame spraying of powdered material, vacuum evaporation, sputtering, and explosive bonding. In some cases, the techniques of the aforementioned patent may result in amorphous coatings, however it is not an object of the invention to prepare amorphous coatings, and, in fact, it appears to be the intention of that patent to return the amorphous coating to a crystalline condition, since the latter patent refers to heating the amorphous films to return them to their crystalline state. Furthermore, no desirable properties or examples of the article thus formed are ascribed to amorphicity or vacuum deposition.
Another process for the production of catalysts for the cathodic hydrogen evolution in an alkaline electrolyte is disclosed in U.S. Pat. No. 3,926,844. This process involves the deposition of amorphous borides of nickel, cobalt or iron by the reduction of their salts in an aqueous bath. While the materials thus prepared are amorphous, and do exhibit some electrocatalytic activity, the method is of limited utility. The range of compositions that can be prepared by this method is quite limited because of the compositional restrictions imposed by the process conditions involved. While low overvoltage is discussed, it does not appear that the overvoltage is in the range of the low overvoltage of the present invention and the only operating examples given are for a temperature of 20.degree. C. which is well below general industry operating temperatures which are in the range of 70.degree. C. to 120.degree. C. and very commonly 80.degree. C. to 90.degree. C. This particular attempt at utilizing a material not having a substantially crystalline structure did not provide a cathode catalyst which has been accepted for commercial use to any significant degree. Since overvoltage drops with an increase in temperature, the lack of higher temperature results, would appear to indicate degradation of the material at the higher temperatures at which the material would be utilized.
In summary, the field relating to catalytic materials for electrolytic cell cathodes has been generally predicated on substantially crystalline materials. Of such materials, those which are capable of withstanding an industrial environment, such as mild steel and nickel, have catalytic inefficiencies which result in relatively high overvoltages adding significantly to operating costs. Those materials which exhibit lower overvoltages than mild steel and nickel, such as noble metal catalysts, are expensive and/or subject to poisoning or degradation. Thus, there remains the need for a stable, low overvoltage cathode material of low cost to replace the presently used cathode materials for hydrogen evolution in an electrolytic cell.